Finishing operations are typically performed on a workpiece in order to alter the surface of the workpiece. The two primary processes for finishing are abrading and polishing. Abrasion refers to the removal of larger portions of the surface, primarily to alter the overall contour of the surface. Abrasion is often performed in a wet process, and may take the form of a grinding, deburring, aggressive smoothing or similar material removal operation. Polishing, on the other hand, refers to the removal of small portions of the surface of a workpiece, in a scratch like manner. The polishing process is intended to primarily alter the visible finish of the workpiece surface. Polishing is often performed in a dry process. The term "finishing" is generally used to refer to both surface abrading and surface polishing as described above.
It is not uncommon for a finishing operation t o incorporate both an abrasion process and an polishing process. Problems, however, may arise when switching from the wet abrasion process to the dry polishing processes. For example, the workpiece must be cleansed before the workpiece can be polished.
Another drawback with conventional automatic (non-manual) finishing operations is that they typically involve tumbling or vibrating the workpiece in a tub containing abrasive media which is not suitable for delicate articles such as semiconductor wafers.
A further problem associated with conventional finishing methods is the buildup of "fines", which are produced during the finishing process by attrition of the finishing media and/or material of the workpiece being finished. Buildup ofthe fines on the abrasive media tends to shorten the useful lifetime of the media. Also, due to their small size and/or tendency to adhere to the workpiece, the fines make cleaning of the finished workpiece difficult. The fines must also be disposed of, which can lead to environmental concerns.
Conventional finishing operations are also not suited for finishing irregular shaped surfaces. Recessed areas of the workpieces often cannot be finished to the same extent as exposed surfaces, thus leading to surface inconsistencies.
One prior art method for polishing or surface abrading irregular articles is described in U.S. Pat. No. 2,735,232. That method employs a mixture which consists of an abrasive powder, a magnetic powder and a liquid which may be any type of lubricating oil. After introducing a workpiece into the mixture, a two or three-phase magnetic field is applied to the mixture which causes the particles to move in small circular or spiral paths, abrading the surface ofthe workpiece as they contact it.
Another known method for grinding surfaces using a magnetic fluid containing abrasive grains is disclosed in U.S. Pat. No. 4,821,466. That method involves placing abrasive grains and an floating pad within a magnetic fluid. A magnetic field is applied to which creates a buoyant force under the abrasive grains and pad. The result is the formation of a high-density abrasive layer. The workpiece is then brought into contact with the abrasive layer and rotates by an external source to grind one surface of the workpiece. The main drawbacks with the system disclosed in U.S. Pat. No. 4,821,466 are the requirement of an external driving force to rotate the workpiece, and the inability to polish all the surfaces of the workpiece at the same time.
While these prior art finishing processes provide some degree of surface finishing for an irregularly shaped item, they are not very efficient and do not provide consistent results.
A need, therefore, exists for an improved finishing process which can be used to finish any shaped item quickly and efficiently.